The present invention relates to metal matrix composites (MMC) manufactured by methods of powder metallurgy especially by infiltrating a loose sintered solid metal powder with low-melting liquid metal or alloy. More particularly, the invention is directed to MMC containing at least one component (solid powder or infiltrating melt) based on lightweight metals such as titanium and magnesium.
Metal matrix composites manufactured by infiltrating with molten metal are attractive materials for structural applications not only due to their excellent properties such as stiffness, light weight, high abrasion and oxidation resistance but mainly due to the opportunity to compose materials containing combinations of metals that can be difficult or cost prohibitive when produced by methods of conventional metallurgy and machining.
However, infiltrated MMC (IMMC) are usually brittle, weak at high temperatures, and exhibit insufficient flexure, fatigue, and impact strengths. Low dynamic mechanical properties limit the application of IMMC especially in aircraft, automotive, and rocket industries.
Various processes have been developed during the last two decades for the fabrication of lightweight IMMC with desirable mechanical properties including vacuum infiltration, low-pressure casting, liquid-phase sintering and self-propagating combustion synthesis among others. All of these new processes as well as conventional powder metallurgy techniques impose certain limitations with respect to the characteristics of the produced IMMC.
For example, JP 60070143, 1985, describes the process of formation of a titanium-magnesium composite by compacting a porous titanium block immersed with the molten Mg at high pressure. The high pressure is needed to provide infiltrating due to a lack of wetting titanium by molten Mg. The porous Ti block should have a relatively high density to be able to hold the high pressure processing, so the block has low porosity. Low porosity limits the amount of infiltrated Mg and therefore limits the ultimate weight decrease. Besides, low porosity results in the incomplete infiltration of small porous channels by magnesium. Even long holding time does not help, and the final structure has a remaining porosity in the central part of the block. This randomly distributed porosity and heterogeneous structure of dense titanium skeleton decrease mechanical properties of the obtained IMMC, especially impact strength and toughness.
From the other hand, the high pressure infiltration of ceramic matrix composites (German patent 19917175, 2000) allows the use of a relatively high level of porosity (xcx9c35%) that results in complete filling of pores by molten metal. But ceramic matrix composites have significantly lower elasticity and impact strength than the projected IMMC. So, the degree of IMMC porosity must be in the controlled range that is not provided by conventional technique.
Besides, it is technically difficult to organize high pressure evenly distributed on the area of large size porous blanks such as plates, sheets, and the like.
The addition of infiltrating enhancers such as ZnO or MgN, as disclosed in U.S. Pat. No. 5,311919, is effective in speeding up the infiltration process and filling out all cavities in the compacted Ti block. However, this method does not improve any mechanical characteristics of the final product due to oxide or nitride intrusions acting as stress concentrators in the composite microstructure. The U.S. Pat. No. 5,890,530 describes the Mg infiltration process in the presence of an oxygen-binding material such as carbon or graphite. This method is not effective if the infiltrated compact is manufactured from titanium or zirconium as they are oxygen-active metals themselves. Besides, carbon-based additives also deteriorate microstructure of IMMC.
The applied vacuum improves the infiltration conditions of molten Al, Mg, Ni, and other metals into a porous ceramic compact under a vacuum of  less than 10xe2x88x926 torr as it is reported in the U.S. Pat. No. 3,718,441. However, the absence of pressure does not produce the beneficial effects on the composite structure as well as the high pressure in the above mentioned methods.
Several attempts have been tried to improve a spontaneous infiltration by alloying Mg-based melt with such elements as Al, Zr, or Zn, for instance, in EP 765946, 1997, and WO 9117278, 1991, or JP 01279715, 1989, or JP 09263858, 1997. The infiltration and consequently the density of composites were improved but mechanical properties remained at the same low level because the structure of the final composites were not really changed.
The MMC based on Tixe2x80x94Ag and Tixe2x80x94Cu compositions were not manufactured by spontaneous infiltration or infiltration under gas pressure. High temperature of the infiltration process caused by high melting point of silver and copper result in their active reaction with titanium and formation of monolith casting structure. That""s why, those compositions are used as brazing filler metals for titanium joining.
Some specialized technologies were offered to manufacture MMC structure without the formation of casting structures, e.g., (a) small amount of silver powder is mixed with Ti powder and sintered together at the temperature over the melting point of silver (as in U.S. Pat. No. 5,983,507), or (b) coating titanium powder by copper and sintered at the temperature over the melting point of copper (as in U.S. Pat. No. 4,381,942). Both those technologies can not provide fully dense uniform structure of MMC, and therefore, can not provide a stability of mechanical properties of the composite materials.
The infiltration of sintered titanium powder preform with low-melting metal, e.g., lead, at 600xc2x0 C. (as described in the U.S. Pat. No. 6,287,433) results also in residual porosity and insufficient strength of the obtained composite plates.
All other known processes of making IMMC have the same drawback: the irregular structure (consisting of the sintered skeleton, casting infiltrate, and residual pores) results in low mechanical properties of the composite.
It is therefore an object of the invention to form an homogeneous, essentially uniform structure of the metal matrix composites providing significant increases of such mechanical characteristics as elongation, toughness, flexure and impact strength or fatigue resistance.
Another object of the present invention is control of the IMMC structure by the formation of the predetermined structure of the compacted preform, and then, a texture in the infiltrated composite that will allow for the control of mechanical properties in the final product.
It is yet another object to provide complete wetting of the surface of titanium powder in the skeleton structure during the infiltration with the Mg-base alloys, Ag- and Cu-based alloys, or Pb to achieve full density of the composite material.
It is still another object of the invention to generate intermetallic compounds on the interface of the titanium matrix and the infiltrated alloy and to achieve the effect of strengthening of the final microstructure by the intermetallics.
A further object of the invention is to ultimately provide for articles, esp. metal composite foils, plates and sheets, that are fully dense and characterized by high mechanical properties.
It is yet another object of the invention to generate low thermal expansion materials with heat-dissipating structure for electronic substrate applications.
The nature, utility, and further features of this invention will be more clearly apparent from the following detailed description with respect to preferred embodiments of the invented technology.
The invention relates to the manufacture of metal matrix composites by loose sintering titanium, titanium aluminide, or titanium alloy powder in the lightweight skeleton structures and by infiltrating them with a molten metal. While the use of Mg-based infiltrates has previously been contemplated in the composite production, as mentioned above, problems related to insufficient wetting, defective microstructure, residual porosity, and low mechanical properties of Ti-matrix composites have not been solved.
The invention overcomes these problems by (1) loose sintering of Ti-based powder to obtain the skeleton structure having a density of 25-35% or sintering of direct powder rolled strip to obtain the skeleton structure having a porosity of 35-60%, (2) deformation of said skeleton structure before the infiltration to obtain a preform having density of 45-90% with the predetermined shape and size of pores, (3) alloying magnesium with Al, Ti, Si, Zr, Nb and V, (4) modification of Mgxe2x80x94Al-based melt with sub-micron particles of TiB2, SiC, or Si3N4, (5) infiltration with Mgxe2x80x94Al-based melt at 450-750xc2x0 C., or with molten In, Pb, or Sn at 300-450xc2x0 C., or with molten Ag and Cu alloy at 900-1100xc2x0 C. followed by (6) rolling, die pressing, CIP, cold and preferably, hot deformation, to refine and transform casting microstructure of the infiltrated metal into the deformation microstructure strengthened by intermetallic phases such as TiAl, TiAl3, and Ti3Al, and (7) final re-sintering or diffusion annealing.
In another aspect of the invention there is provided a technology to manufacture fully dense flat or shaped lightweight construction articles based on Tixe2x80x94Mg, Tixe2x80x94Mgxe2x80x94Al, or Tixe2x80x94Pb, Tixe2x80x94Ag, and Tixe2x80x94Cu infiltrated metal matrix composites.
In essence, the core of the invention is to control the composite microstructure using (a) loose sintering, (b) customized deformation before and after the infiltration, (c) alloying or modifying the infiltrated metal, and (d) heat treatment realizing dispersion-strengthening. The controlled microstructure results in significant improvement of mechanical properties of the composite material.
The method allows the control of the microstructure of the composite materials by changing parameters of deformation, infiltration, and heat treatment. The method is suitable for the manufacture of flat or shaped metal matrix composites having improved ductility such as lightweight bulletproof plates and sheets for aircraft and automotive applications, composite electrodes, heat-sinking lightweight electronic substrates, as well as for sporting goods such as helmets, golf clubs, sole plates, crown plates, etc.